Variable displacement axial piston hydraulic units utilize a tiltable swashplate to control the displacement of pistons reciprocatably positioned within a rotating cylinder barrel. Fixed displacement axial piston hydraulic units use a fixed swashplate to define the units displacement. Each of the pistons commonly have a protruding end swivably connected to a slipper seated against a cam surface on the swashplate. One of the problems encountered therewith is that during operation the slippers are sometimes subjected to forces tending to lift the slipper pads off the cam surface. Such liftoff forces are generated by centrifugal forces and low inlet pressures that create suction at the top of the piston during the inlet portion of the stroke.
This problem is generally solved by the use of a holddown spring which resiliently biases the slippers toward the cam surface through a holddown plate. Belleville springs are widely used as the holddown spring since they can generally create relatively high loads in a fairly short axial length. However, the disadvantage of using Belleville springs is that they usually require stacking several Belleville springs in a predetermined sequence thereby introducing the likelihood of assembly errors. Another problem is that the spring force exerted by Belleville springs are concentrated in a narrow, annular contact band at the edges thereof. This leads to premature wear to the contact bands which quickly causes a shortening of the axial lengths of the stack of springs. Shortening the length of the stack of Belleville springs greatly decreases the holddown forces exerted on the slippers.
In view of the above, it would be desirable to have a slipper holddown device that utilizes a single spring for minimizing assembly errors while providing relatively high loads in a small package without concentrating the forces in a relatively narrow contact band.